The invention relates to circuits and methods for FM signal discrimination, particularly for discriminating FSK (frequency shift keyed) signals in modem receivers.
Modems are commonly used to allow reliable communication of digital data over telephone lines. FSK techniques are used, wherein two separate frequencies, a low frequency (for example, 1070 Hz) and a high frequency (for example 1270 Hz) are used to represent two logic levels, respectively.
Acoustic couplers are commonly used in conjunction with modems to couple a computing system to an ordinary telephone. Telephone microphones usually are carbon microphones, and they are known to produce a substantial amount of harmonic distortion. The modem communication frequencies (which are chosen by the telephone company) and the harmonic distortion frequencies produced by carbon microphones in ordinary telephones are such that the harmonic distortion produced by acoustic coupling of digital data to the telephone line can fall within the same band as the two FSK signal frequencies representing digital data. The FSK receiver of the modem often has difficulty discriminating the two meaningful predetermined FSK signal frequencies from the accompanying harmonic distortion frequencies produced as a result of non-linearities in the above-mentioned carbon microphones.
Usually "digital" discriminating techniques have been used to perform the function of distinguishing the two desired FSK signal frequencies from the undesired harmonic distortion frequencies which lie therebetween. Unfortunately, the known digital FM discriminating techniques all perform their function by "looking" at zero crossings of the signal to be discriminated. Consequently, "jitter" of the zero crossing points caused by the above-mentioned harmonic distortion often results in discriminating errors. To avoid such errors, it has been necessary for prior art discriminating circuits to test at least a few successive zero crossing points to be sure they are all in the same frequency range before a decision is made as to whether the signal frequency represents a "1" or a "0". This requirement leads to increased circuit cost and complexity and decreased circuit speed. This is highly inconsistent with the growing need for higher speed, lower cost digital communication via telephone systems.
Thus, there remains an unmet need for a system for avoiding data errors caused in FSK receivers by harmonic distortion produced by telephone carbon microphones operating in conjunction with acoustic couplers coupled to the modem receivers. More specifically, there remains an unmet need for a system for economically and accurately discriminating between two closely spaced FM signals of predetermined frequencies despite the presence of noise signals having frequencies that lie between the two predetermined frequencies.
Therefore, it is an object of the invention to provide an apparatus and method for accurately and economically discriminating between first and second FM signals having different first and second predetermined frequencies, respectively, despite the presence of noise signals with frequencies between the first and second predetermined frequencies.
It is another object of the invention to economically avoid the errors caused in prior FSK receivers by harmonic distortion signals produced as a result of non-linearities in telephone carbon microphones.